The present invention relates to a material for packing containers, this material comprising a carrier layer of cardboard or paper hereinafter also generally referred to as "paper material", inner and outer layers of thermoplastics and a layer of aluminum foil, selected portions of the material being intended through overlap-folding (so-called z-folding) and sealing together of the overlapping portions to produce reinforced regions or panels comprising three material layers sealed together. The invention also relates to the method for manufacturing the material.
In packaging technique packages of the non-returnable type have been used for a long time which are manufactured from a material which consists of a carrier layer of cardboard or paper and outer and inner coatings of thermoplastics. Frequently the packing material in such packages is also provided with layer of other materials, e.g. aluminum foil which is a gas-tight material and which makes possible sealing with the help of a high-frequency electromagnetic filed. In this method of sealing it is the objective to generate in the aluminum foil layer electric induction currents which heat the aluminum foil layer and through conduction transmit heat to the adjacent thermoplastic layer which constitutes the sealing layer of the material. The thermal energy required for the sealing operation is thus achieved in two stages, a first stage in which the aluminum foil layer is locally heated with the help of induction currents and a second stage where the heat of the aluminum foil layer is transmitted by thermal conduction to the thermoplastic sealing layer.
The composition of a packing material has the purpose of creating the optimum product protection for the goods which are to be packed, at the same time as imparting to the package sufficient mechanical protection for the product and adapting it so that it can be readily handled by the user of the package. To achieve mechanical rigidity which on the one hand provides mechanical protection and on the other hand makes it possible for the package to be of such rigid form that it can be handled and gripped by hand without difficulty, the packages of this type are often provided with a carrier layer of paper or cardboard. Such a carrier layer, however, has poor tightness characteristics in respect of gases or liquids and the good rigidity of the material disappears if the material is subjected to moisture which is absorbed into the material. To impart a good liquid-tightness to the material it is often laminated with a plastic material and if this plastic material is thermoplastic the plastic layers can be used in the manner mentioned previously to seal the plastic layers to each other with the help of heat and pressure, and in this manner the packing container can be sealed and made permanent in its given form by sealing the overlapping or adjacent material panels which are thermoplastic-coated to each other in a tight and durable seal.
Packing containers of the type referred to here are manufactured either from blanks punched out beforehand or from a continuous web which has been provided with suitable decoration and with a crease line pattern facilitating the fold-forming. The packing containers are manufactured from such a web by joining together the longitudinal edges of the web in an overlap joint so as to form a tube which is subsequently filled with the intended contents and divided into closed container units by means of repeated transverse seals of the tube perpendicularly to the longitudinal axis of the tube. After suitable fold-forming of the packing material in the tube the material in the said container units is converted to the desired geometrical shape, usually a parallelepiped, by providing the tube with longitudinal folding lines and with double-walled triangular lugs at the corners of the packing container. Whether packing containers are manufactured from blanks produced beforehand or from a continuous web, the material, for practical reasons, will be of uniform thickness. This means that the material, and in particular the carrier layer of paper or cardboard, is mechanically overstrong or overdimensioned along certain regions which during normal use and handling are not subjected to major mechanical stresses while other parts of the material in the package ought to be thicker or more rigid in order to withstand the mechanical strains which they are subjected to. In other words it would be desirable for the material to be of a differential thickness so as to achieve a better economic utilization of the material.
It is known that such a material with differential thickness can be produced by folding the material to form a double fold (a so-called z-folding), the overlapping portions of the material subsequently being sealed together to a rigid wall part of threefold material thickness. It is also known from Swedish patent application No. 8405539-1 that certain portions of such packing material can be thickness-reduced to make possible the folding and sealing of the material.
In the case where the sealing is realized with the help of a high-frequency electromagnetic field which induces currents within a selected sealing region such a sealing cannot be carried out on a z-folded material since this material in the region of the z-fold will have three material layers arranged on top of one another which, even if they are thickness-reduced, will each have an aluminum foil layer. This means that the outer coils generating the magnetic field which are used for creating the desired induction currents in the aluminum foil will generate an induction current in the aluminum foil layer situated closest that is to say the outer one while no current, or only negligible current, will be generated in the two subjacent aluminum layers, the innermost of which is that which is in direct contact with the thermoplastic layer which ought to be heated to sealing temperature. Hence it is not feasible in a sealing region to arranged several parallel aluminum foil layers situated on top of each other but the aluminum material in the two upper material layers in the z-region must be removed within the sealing region, since otherwise no sealing heat will be transmitted to the sealing region.